Overload protector



s P 1950 M. c. HARROLD 2,521,120

OVERLOAD PROTECTOR Filed Dec. 11, 1946 66 B/METAL I45 u I44 I 2 LIVENTO:

BY CM Patented Sept. 5, 1950 UNITED STATES PATENT OFFICE corporation ofDelaware Application December 11, 1946, Serial No. 715,477

6 Claims. (Cl. 318-2'21) This invention relates to electric apparatusand more particularly to overload protectors for electrio motors.

Where motor overload protectors are self-reing it is customary to setthe protector to trip at a sufiiciently low current flow to properlyprotect the motor windings for one or several resettings and yetsufficiently high to permit a generous amount of overload. Shouldstalling conditiOnS T eavy overloads continue for extended '10 periodsof time the protectors will reset repeatedly. The customary settingpermits each successive how of current upon each resetting to increasethe temperature of the windings so that eventually the temperaturebecomes high enough to damage the insulation.

It is an object of my invention to provide an overload protector inwhich the settings are sufficiently high to obtain full output of themotor and in which the heating of the motor is re-' duced as theprotector continues to trip.

It is another object of my invention to provide an overload protector inwhich the closed period of the protector is reduced or the open periodof the protector is prolonged or both, as 335 the stalling or overloadcondition continues.

It is another object of my invention to use the rise in temperature ofthe starting control to control the reduction of the proportion orclosed time to open time of the protector as stalling or overloadconditions continue.

These objects are attained by placing the thermal element of theoverload protector in heat exchange relation with the electromagneticstarting control so that as the protector continues to trip and resetand the motor and starting control rise in temperature, the proportionof closed to open periods of the protector will be reduced. Furtherobjects and advantages of the present invention will be apparent fromthe following description, reference being had to the accompany-Referring now more particularly to Fig. 1, there is shown, for thepurpose of illustrating one application of my invention, a capacitorstart and run motor control system including an electric motor 20 havinga. main winding 22 and a phase winding 24. The common junction of themain 2 and phase windings 22 and 24 is connected by a switch 26 to thesupply conductor 28. The other end of the main winding 22 is connectedby the conductor 30 to the starting relay electromagnet coil 32 which inturn is connected by the conductors 34 and 36 to the actuating bimetal38 of the motor overload protector 40. The actuating biinetal 38 carriesat its free end a, contact which is adapted to make contact with astationary contact 42. The stop screw 44 limits the extreme open circuitposition of the 'bimetal 38. A compression spring 46 extends between thefree end of the bimetal 38 and an anchorage 48 to cause the bimetal 38and its contact to open and close with a snap action. The bimetal 38 maybe completely self-heated or it may be both selfheated and externallyheated by the electric heator 50 connected between the stationarycontact and the second supply conductor 52. I

The other end of the phase winding '24 is connected by the conductor 54to a permanent capacitor 56 which in turn is connected by the conductors58 and 60 to the conductor 36 which through the overload protector 40connects with the supply conductor 52. This same end of the phasewinding 24 is connected by the conductor 62 to a stationary contact 64mounted upon the supporting frame 66 which supports the electromagneticcoil 32. The armature 6B of the electromagnet 32 is connected to a leaispring 10 carrying a movable contact adapted to make contact with thestationary contact when the electromagnet 32 isenergized and thearmature 68 is lifted. I This closes thecircuit through the conductor 12to the startingcapacitor which connects through the conductor 16, andthe conductors 60 and 36 with the overloadprotector 40 which in turn isconnected to the supply conductor 52. As soon as the motor 21] startsand attains sufiicient speed the current flow through the electromagnet32 becomes sufiiciently small so that the armature 68 drops andseparates the movable contact from the stationary contact 64 todisconnect the starting capacitor ID from the phase winding circuit.

Should an overload occur, the bimetal 38 will be heated by the currentflow through it and the heater 50 and will bow upwardly to lift themovable contact from the stationary contact 42 to disconnect the systemfrom the supply conductor 52. The upward movement of the bimetal so willbe stopped by the stop screw 44. The position of the stop screw '44 willdetermine the temperature to which the bimetal 38 must cool before itwill return to the closed circuit position shown in Fig. l. The settingof the stop screw depends upon the application of the m tor 28. For someapplications a long off period is permissible but for many others theoff period should be as short as possible so that operation can beresumed with a minimum of delay.

Under the latter condition it will often happen that if there is acontinued heavy overload or a stalling condition in which the motor isprevented from rotation, the current flow will be so heavy as to causethe overload protector to trip within a very short time. Since theprotector is self-resetting it will reset regardless of the continuedcause of the tripping and at each resetting a considerable amount ofcurrent will flow through the windings of the motor 28. Each time theprotector resets current flows through the motor dissipating a certainamount of energy in the motor to heat the motor until the protectoragain trips. If this is permitted to continue for a sufficiently longperiod of time the motor windings will become sufiiciently hot to damagethe insulation.

According to my invention, instead of anchoring the bimetal 38 upon anyconventional support, I fasten the end of the bimetal 38 directly totheU-shaped support 65 for the electromagnet 32 by some suitablefastening means so that the anchored end of the bimetal 38 is in directheat exchange relation with the electromagnet 32. Since theelectromagnet 32 heats up in the same way as the electric motor undercontinued stall or overload conditions, the bimetal 38 will be heatedfrom the heat generated by the electromagnet 32 in proportion to theheating of the motor 23 during each starting period. This heating by theelectromagnet 32 will be in addition to the self-heating and/ orexternal heating usually provided as, for example, shown in Fig. 1. Withthis arrangement the electromagnet 32 is heated and becomesprogressively hotter because of the heavy current fiow during continuedstalling and overload conditions occurring during the starting. period.The bimetal 38 will derive heat from the electromagnet to trip at alower current value and to remain open for a longer period before theresetting temperature is reached. Thus by this arrangement the trippingis accelerated and the resetting is retarded.

In Fig. 2 is illustrated another form of the invention in which the samemotor, motor circuit and starting control are used and bear the samereference characters. The overload protector, however, is difierent andincludes an external heater I50 connected at one end to the supplyconductor I52 and at the other end to the anchorage of the actuatingbimetal I38. The free end of the bimetal I38 carries a movable contactwhich is adapted to contact the stationary contact I42 connected by theconductor I31 with a conductor I extending to one end of theelectromagnet coil 32. The reset stop screw I 44 is mounted upon the endof the bimetal strip I45 which is anchored to and is in heat exchangerelation with the frame 66 of the electromagnet coil 32 and derives heattherefrom with the rise in temperature of the electromagnet 32.

A toggle spring I46 extends between the free end of the bimetal I38 anda support I48 which may be mounted upon the frame 68. The toggle springI46 causes the bimetal I38 to move to open and closed positions with asnap action. As mentioned in connection with Fig. 1, the position of thestop screw I44 determines the temperature at which the bimetal I38 willreset. If the screw I44 is moved closer to the stationary contact I42the bimetal I38 will reclose at a higher temperature. The bimetal I45,however, is so arranged that as it rises in temperature due to theheating of the electromagnet 32 it will curl upwardly to move the stopscrew I44 away from the stationary contact I 42 so that it will takelonger for the bimetal I38 to cool to the lower temperature it mustreach before it is reset. The longer the stalling or overload conditioncontinues the higher will be the temperature of the electromagnet 32.Thus as the stalling or overload condition continues, the stop screw I44will be moved further away from the stationary contact I42 to prolongthe open circuit period of the overload and to reduce the proportion ofclosed time to open time of the overload protector so that thetemperature of the motor is prevented from reaching dangerous limits.

By either of these arrangements, the heating effect of the currentduring protector controlled overload and stalling conditions is reducedas the condition continues so that the temperature of the motor will besafely limited. While I have shown the invention as applied to acapacitor start and run motor, it may be applied to other types ofmotors requiring a starting control, such as, for example, split phasemotors.

While the form of embodiment of the invention as herein disclosed,constitutes a preferred form, it is to be understood that other formsmight be adopted, as may come within the scope of the claims whichfollow.

What is claimed is as follows:

1. In combination, an electric motor having main and phase windings, anelectrically operated starting control for controlling the energizationof the phase winding, a thermal element in heat exchange relation withsaid starting control, self-resetting overload protector fordeenergizing said electric motor upon an overload, said overloadprotector including'a resetting stop connected to and operated by saidthermal element for varying the reclosing of the protector in accordancewith the temperature of the starting control.

2. In combination, an electric motor having an electromagnetic startingcontrol, a thermal element in heat exchange relation with said startingcontrol, a self-resetting overload protector for deenergizing saidelectric motor upon an overload, said overload protector including aresetting stop connected to and operated by said thermal element forvarying the reclosing setting of the protector.

3. In combination, an electric motor having an electromagnetic startingcontrol, said starting control including a metal magnetic circuitportion, a metal thermal motor in direct metal-tometal contact with saidmetal magnetic circuit portion and in metallic heat exchange relationtherewith, a self-resetting overload protector for deenergizing saidelectric motor upon an overload, said overload protector including aresetting stop connected to andoperated by said thermal motor forvarying the reclosing setting of the protector.

4. In combination, an electric motor having main and phase windings, anelectromagnetic starting control for controlling the energization of thephase winding, said starting control including an electromagnet coil anda metal frame of magnetic material fixed to and rigidly connected tosaid coil, a thermal element mounted directly upon said frame directlyadjacent said coil in direct metal-to-metal contact with and in directmetallic heat exchange relation with said frame, means for heating saidthermal element in response to motor current, and selfresetting meansconnected in series with both the main and phase windings and operatedby said thermal element for completely deenergizing said motor upon anoverload.

5. In combination, an electric motor having main and phase windings, anelectromagnetic starting control for controlling the energization of thephase winding, said starting control in cluding an electromagnet coiland a metal frame of magnetic material fixed to and rigidly connected tosaid coil, a self-resetting overload protector connected in series withboth the main and phase windings for completely deenergizing saidelectric motor upon an overload, said protector including metal thermalmeans mounted direct- 1y upon said metal frame directly adjacent saidcoil in direct metal-to-metal contact with and in direct metallic heatexchange relation with said metal frame for reducing the proportion oftime the protector is closed whenever unsuccessful starting attemptscontinue.

6. In combination, an electric motor having main and phase windings, anelectromagnetic starting control for controlling the energization of thephase winding, said starting control including an electromagnet coil anda metal frame of magnetic material fixed to and rigidly connected tosaid coil, a self-resetting overload pro- REFERENCES CITED 'lhefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,491,396 Hall Apr. 22, 19241,637,042 Hutt July 26, 1927 1,763,197 Speiser June 10, 1930 1,785,852Wilms et a1 Dec. 23, 1930 2,037,118 Chubbuck Apr. 14, 1936 2,175,032Shaefer Oct. 3, 1939 2,255,437 Pearce Sept. 9, 1941 2,299,669 WernerOct. 20, 1942 2,311,048 Harrold Feb. 16, 1943 2,379,602 Stickel July 3,1945 2,436,909 Werner Mar. 2, 1948

